The present application relates generally to power distribution systems and, more particularly, to protection systems for electric distribution systems.
Known electric power grids typically include power generation plants, transmission and distribution lines, transformers, and other devices that facilitate electric power transmission, and power delivery. After electric power is generated in the generating plants, it is transmitted for extended distances through the high voltage transmission lines to subtransmission/distribution substations. Transmission lines usually operate at voltage levels between approximately 115 kilovolts (kV) and approximately 765 kV. At the subtransmission/distribution substations, transformers reduce the high voltage at which the power has been transmitted to sub-transmission voltage levels that range from approximately 46 kV to approximately 69 kV, or to distribution voltage levels that range from approximately 12 kV to approximately 34.5 kV. Power is then transmitted through a feeder to an end customer through an electric distribution system, and before it reaches the end customer, the voltage is decreased to approximately 120V/240V by a distribution transformer.
Most known electric distribution systems include a plurality of feeders coupled to the substation transformer. The electric distribution systems may also include at least one capacitor bank, at least one voltage regulator, and at least one distributed generation (DG) device, e.g., a diesel generator and a photovoltaic source. The feeder is divided into smaller units via bus-bars, disconnect switches, reclosers, sectionalizers, and fuses, wherein such smaller units are referred to as segments. Each segment may have any number of DG devices coupled thereto.
Therefore, typically, most known electric distribution systems include a plurality of segments with a plurality of DG devices coupled throughout the segments. In the event that a fault occurs on a segment, DG devices may contribute to the fault current along with the substation. This results in a bi-directional fault current, and traditional relays sensitive to current direction may not initiate protective actions within predetermined specifications. Another issue with connection of distributed generators is that it changes the fault current of the distribution system. In other words, when you connect a distributed generator to the distribution system it will contribute to the fault current based on the power it is generating. This can lead to a failure of protection systems to detect faults when there are high levels of distributed generation. Therefore, conventional/traditional protection schemes/systems can be difficult to coordinate.